1. Field Of The Invention
The present invention relates to a method of controlling the scanning of a thermal head of an image recording apparatus.
2. Background
Among image recording apparatuses designed to record an image on paper wound in the form of a roll or cut in the form of a sheet, there is one type of apparatus in which the entire paper surface having a predetermined size is not recorded simultaneously, but rather the paper is scanned with a recording device over an area corresponding to the main scanning width of the recording device repeatedly until the required recording is complete. For example, an image recording apparatus that employs thermalsensitive paper is arranged such that a thermal head having a predetermined main scanning width is moved in the sub scanning direction in contact with the thermalsensitive paper to thereby effect printing of one line. After each line has been recorded, the thermalsensitive paper is fed one line in the main scanning direction, which procedure is repeated to effect recording on a predetermined sized paper.
Printing by means of a thermal head is effected in such a manner that the thermal head is supplied with a printing signal and moved in the sub scanning direction (in a first direction). After the completion of printing a single line, the printing signal is terminated with the thermal head remaining in contact with the paper. Thereafter, the thermal head is moved in a second direction opposite to the first direction so as to return to the print start position. Accordingly, the thermal head stops once on the thermalsensitive paper when the head, having moved in the first direction to print the thermalsensitive paper, is reversed so as to move in the second direction.
When the printing signal is terminated, the generation of heat in the thermal head is suspended, but the thermal head itself has heat absorbing properties. Therefore, when the thermal head stops on the thermalsensitive paper as described above, the paper is heated twice at the head stop position by both the heat generated in response to the printing signal and by heat retained in the head. Accordingly, the image corresponding to the thermal head stop position becomes darker than the other images, resulting in nonuniform printing density.
To eliminate the nonuniform density, one possibility is to prevent the contact between the thermal head and the thermalsensitive paper after the completion of printing by the head. However, this solution requires a thermal head release mechanism comprising, for example, a plunger, solenoid, cam, etc., which complicates the structure of the recording apparatus and increases the overall size of the apparatus.